feat: 删除提问时级联删除其回答，并移除回答上的删除按钮

- 删除按钮只保留在用户提问气泡上；AI 回答不再单独提供删除
- 后端 DELETE /messages/{id} 改为按整轮删除：从提问删到下一条提问之前，
  返回实际删除的 deleted_ids，保持 checkpoint 合法（无孤立工具结果/回答）
- 前端隐藏后端返回的整批 id，删除后即时消失
- 更新删除相关中英文案

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend-web/src/core/threads/hooks.ts

@@ -805,11 +805,13 @@ export function useDeleteThread() {
 }
 
 /**
- * Delete a single message (a user question or an assistant answer) from a
- * thread. Hits the Gateway endpoint that filters the message out of the latest
- * checkpoint. The live `useStream` cache is not auto-refreshed by this call —
- * callers hide the deleted id locally (see `MessageList`) so the removal shows
- * immediately; a reload then reads the persisted, already-trimmed state.
+ * Delete a conversation turn (a user question together with its answer) from a
+ * thread. Hits the Gateway endpoint that removes the question and every
+ * following message up to the next question from the latest checkpoint, and
+ * returns the ids that were actually removed. The live `useStream` cache is not
+ * auto-refreshed by this call — callers hide the returned ids locally (see
+ * `MessageList`) so the removal shows immediately; a reload then reads the
+ * persisted, already-trimmed state.
  */
 export function useDeleteMessage() {
   return useMutation({
@@ -819,7 +821,7 @@ export function useDeleteMessage() {
     }: {
       threadId: string;
       messageId: string;
-    }) => {
+    }): Promise<string[]> => {
       const response = await apiFetch(
         `${getBackendBaseURL()}/api/threads/${encodeURIComponent(
           threadId,
@@ -835,6 +837,12 @@ export function useDeleteMessage() {
           .catch(() => ({ detail: "删除消息失败。" }));
         throw new Error(error.detail ?? "删除消息失败。");
       }
+
+      const body = (await response.json().catch(() => ({}))) as {
+        deleted_ids?: string[];
+      };
+      // Fall back to the requested id so the caller can always hide something.
+      return body.deleted_ids?.length ? body.deleted_ids : [messageId];
     },
   });
 }